TFTs are widely used as switching elements in drive circuits of various types of devices. For example, in an active matrix type liquid crystal display device, each display pixel is selected by a TFT.
Generally, a TFT has a source electrode and a drain electrode formed to be separated from each other, a semiconductor film formed to electrically contact these electrodes, a gate insulating film formed on the semiconductor film, and a gate electrode formed on the gate insulating film. TFTs of this type can be classified into a staggered type TFT in which a semiconductor film, a gate insulating film, and a gate electrode are sequentially formed on the source and drain electrodes, and an inverted staggered type TFT in which a gate insulating film, a semiconductor film, and source and drain electrodes are sequentially formed on a gate electrode.
In recent years, an increase in ON/OFF current ratio is requested in order to render a TFT operable in a higher frequency range. To achieve an increase in the ON/OFF current ratio, the length of a channel corresponding to the semiconductor film portion between the source and drain electrodes has been decreased. When the semiconductor film of the TFT is made of a silicide semiconductor represented by amorphous silicon (a-Si:H), photocarriers are generated in the semiconductor film upon incidence of light. Although a decrease in channel length can increase an ON current Ion, it facilitates movement of the photocarriers, thereby undesirably increasing an OFF current, i.e., a photoleakage current Ioff. As a result, the ON/OFF current ratio is not improved as expected.
Especially, in an active matrix type liquid crystal display device, generation of the photoleakage current Ioff directly causes variations in potential of the pixel electrodes, leading to a degradation in visual quality of the display image.
Therefore, conventionally, countermeasures have been taken such as formation of a light-shielding layer made of a metal material, e.g., chromium (Cr), on a counter substrate which is arranged to oppose an array substrate on which a plurality of pixel electrodes are formed. However, this cannot prevent the photoleakage current Ioff which is caused by the photocarrier generated in the semiconductor layer of the TFT upon incidence of light reflected by the surface of the light-shielding layer.
According to another countermeasure, for example, a light-shielding layer adjacent to the upper or lower portion of the TFT may be provided on the array substrate. However, this countermeasure cannot essentially decrease the photoleakage current Ioff sufficiently.